A friend of mine is with Optus and I had to Port Forward for her and there where no DNS numbers listed....
So I rang Optus and after argueing with the customer service operator for at least 30 mins, I asked to speak to his supervisor and after arguing with him about the modem having a Nat Firewall he gave in and said I will ring you back...Well sure enough he rang and gave me the 2 DNS numbers I originally asked for and I then successfully Port Forwarded...
21 gigs I envy you lol...I only get 12 gigs a month for $70...
anyway enough of my ramblings...With your download and upload speeds are they in KB/s or in kbps...
Did you do an official speed test ??
Speedtest.net - The Global Broadband Speed Test